Protection Coordination in AC Microgrid via Novel Voltage-Supervised Directional Over-Current Relays

Conventional protection schemes are susceptible to dynamic changes in AC microgrids having diverse Distributed Generators (DGs). These changes, caused by different DG types, significantly alter the fault current levels, leading to conventional Directional Over-Current Relays (DOCRs) miscoordination. The feasibility of using a single set of settings in conventional DOCRs for Grid-Connected Mode (GCM) and Islanded Mode (IM) of operation of AC microgrid is often hindered in the presence of inverter-interfaced DGs due to relatively low short-circuit levels in IM. Hence, this paper uses a novel local voltage measurement-based pickup setting of DOCRs, where the relay operating time is a function of relay voltage and current. A protection coordination problem is also solved to obtain a single set of optimal settings for these DOCRs, which are feasible for various operating scenarios, including GCM, IM, line, and DG outages in an unbalanced AC microgrid. Simulations for different faults in the modified IEEE 13-bus network with synchronous and inverter-interfaced DGs and comparison with previous works validate the reliability and reduced relay operating times in the proposed method. The Controller Hardware-In-Loop (CHIL) simulation using the Real-Time Digital Simulator (RTDS) and dSPACE DS1104 DSP board validates the proposed method's real-time efficacy.